1. Field of the Invention
Our invention relates to the preparation of graft copolymer compositions which absorb up to about 2,000 times their weight of deionized water and proportionately large quantities of other aqueous fluids.
2. Description of the Prior Art
Polymeric substances which possess the ability to absorb aqueous fluids are known in the prior art. For example, U.S. Pat. Nos. 3,669,103 and 3,810,468 disclose that a variety of monomers may be polymerized, with crosslinking, to give polymeric absorbents. The crosslinking reaction is of critical importance, since the noncrosslinked polymers are water soluble and thus have no utility as absorbents.
Water-absorbing alkali metal salts of saponified granular starch-PAN graft copolymers are disclosed in U.S. Pat. No. 3,661,815. In this disclosure, starch is graft polymerized in the granule state, and the saponification is carried out in an alcohol-containing medium to obtain a granular insoluble absorbent. U.S. Pat. No. 3,932,322 discloses a mixture of the composition of U.S. Pat. No. 3,661,815 with fumed silica or alumina. This mixture exhibits an increased rate of fluid uptake and a decreased tendency toward dusting.
Water-absorbing alkali metal salts of saponified gelatinized starch-PAN graft copolymers are disclosed in U.S. Pat. No. 3,935,099, herein incorporated by reference. In this disclosure, starch is gelatinized by heating in water prior to graft polymerization; also, the graft copolymer is saponified in water to give a viscous dispersion of highly swollen but still insoluble microgel particles. Contrary to the absorbent composition of U.S. Pat. No. 3,661,815, the composition of U.S. Pat. No. 3,935,099 may be dried to a continuous film which has an unusually high absorbency for aqueous fluids. Moreover, this film-forming tendency permits a variety of substrates to be coated with thin films of the absorbent composition and thus leads to dramatic increases in fluid absorbencies of the substrates.
U.S. Pat. No. 4,045,387 discloses highly absorbent polymeric compositions prepared by essentially the same process disclosed in U.S. Pat. No. 3,935,099 except that flour is substituted for the starch. These flour-derived absorbents have higher water absorbencies than the corresponding products derived from starch. However, like the starch-derived products of U.S. Pat. No. 3,935,099, these compositions require saponification of the graft copolymer to convert the hydrophobic synthetic polymer moiety to a hydrophilic entity which is capable of absorbing aqueous fluids. This step is time consuming and involves the use of a considerable amount of energy as heat in order to drive it to completion. Moreover, these prior art compositions become substantially nonabsorbent at pH's less than about 4.